Method for preshrinking crystalline vinylidene chloride copolymer film



- R. METHOD FOR PRESHRINKING CRYSTALLINE VINYLIDENE -CHLORIDE coPoLYMER FILM Filed Aug. 1, 1949 July 22, 1952l D. LQWRY ET AL 2,603,838

Up fo /85 Speed@ (/n if .IN V EN TORS. Roer/ Q Lowry BY Haya L. Sc/ 061%/- /4 TTOR/VE YS shrinking crystalline vnylidene chloride copoly- Patented July 22, 1952` cm1-ED ,STATES METHOD Fo-B, PRESHRINKING CRYSTAL- f y LINE VINYLIKDENE cHLoRmEooroLyl MER FILM Robert D. Lowry, Winchester, Mass., ,fand HugoV L. Schaefer, Midland, Mich., assignors to The Dow'Chemical Company, Midland, Mich., 'a cor;

poration of Delaware ,ApplicationAugust1,1949, sesamo'.107,958'- f sjclaims. (c1.1s-4s)j This invention' relates to. .a method for pre.

mer iilm Whereby the lm is rendered dimensionally stable at temperatures up to 212 F.

It has been shown by Wiley, in U. S. Patent No. 5 2,233,442, that those copolymers of vinylidene chloride which contain up to lper cent or so of a copolymerized monoethylenically unsaturated monomer are often crystalline, as determined by the X-raydiffraction patterns of such lo copolymers. VIt has also beenshown that such normally crystallinecopolymers ofvinylidene chloride may be formed into useful oriented crystalline articles by ap'rocess including the successive steps of melting the copolymer, shaping it, cooling it rapidly to a temporarily non-crystalline, supercooled state, and `'cold-stretching the supercooled article. These properties have been utilized commercially in the production of strong, moisture-impervious flexible films by the process described. by ,Stephenson in U. QS. Patent No. 2,452,080. In the Ylatterprocess', the copolymer isI melted and extruded downwardly through a tubeforming orice into acold Water bath, Where it isv supercooled.` The supercooled tube i s withdrawn' 25 from the bath andis'inflated With air toeiect a 3- to 5-fo1d radial distension and :a2-j to 4-fold longitudinal stretch.` The resultingfu'oriented crystalline film may be used inits'v 'tubular' formr or it may be slit and opened to produce a`iia`t lm. y

The lm resulting from Stephensons process is a useful article of commerce on account of its high tensile strengthfinl both principaldirections,

its flexibillity, andespecially because of its extreme resistance to permeability by Water, Water l vapor and other liquids andvaporslA ItY is lused extensively for Wrapping foods and other materials where it is desired either 'to'retain nat1r'al`` Y, I

Istabilized continuously, Iand-under tension, over moisture in or to. exclude it from'the. Wrapped p2 article. y

It has been observed thai-.the oriented .crystalline vinylidene y chloride copolymer lms' discussed above have a tendency toshrink irregularly on standing at room temperature, even :when under the tension existing in tightly wound rolls @145' of the nlm in storage. Such wound lms tend to shrink laterally,Y producing an lirregular Wrinkle effect in the lm'when it is subsequently unwoun'df f for use. The retractionwhich[occurs` Von stand-f I ing, coupled with the different degrees Qf, Stretchf ing of the lm radially and'long'itudinafllt during' its production, gives a nlm `With,'jhighly'{ii'regularA and unpredictable shrinkage characteristics). f when subjected to' elevated {temperaturesf It is,A y common experience to iinfd' that orientedfilms" 55; shrink when heated, and, 4'."ys'lrlfieri,such shrinkag'ef is regular'and uniform it is used GQ advantage in" c other `11an d,:u a nim, shrinks `irregularly 'when exposedv toheat, it cannot be used rsucces''svfully'to lems Vare avoide i 2 certain types of packaging y operations.,

produce neatly packaged ,l articles Whenever-la heat-seal is Arequired Ain Vthe 'yvrappingfopration The welding of superposed ,layers ora" thermally unstable lm causes 'unsightlylwrinkles and un? even tensions alongthe'yveld line. Va'riojis. jtherff mal treatmentsl have beenA proposed iorfp' 4 shrinking orientedlms so aste avoi'dformin" mize these difficulties, but! none has', had suif'vv cient manipulative flexibility" to` be 'practical. Laborious tests on smallpieces oijflrn have srlioyvn'j that, if the lfilm can vb'ef processeciinIsuoli adrian# ner that subsequent. exposure forabout 10i min'fl utes to temperatures 'up to 100'C;resultsA f more than 5 per centshrinkage, vthe s tated'prob# d. andwhgnroduts' page..

for the mesieracting,\1,--l= s.4.'V

It isV an object 'of the/presntiinvention mtoyfpr vide 'al method fcrjpres l crystalline'vinylidene improve *its react" C' ment ,to a point' at vvliich'feliips'ur-for'ilOl-m'ii-f utes to ,atemperature' of 212 res'ult's'finshrink@- age oi'not'to xfceed'percet al ng' it dimension of,I'the'niiln'i.1 vide s uch a inethd Whic "frnay a part v'ofthe prdcssirr vvliiel''hthe lrrr l1s-first produced. Another object is to provide a continuous Aprocess.Afor,..preshrinking and thermally stabilizing the lrn, either as an adjunct of the i original nlm-:formingoperations or to operate independently Longa simply oflm previously produced. Related objects-may appear hereinafter. l The method'oi the present invention, whereby Vfthe oregoing Yand"rfelated"b'j'ecfts are realized,

,cornprises"advanc'ingleJ lsheetof lthe lm to be a series of ati least-four rotatingjsmooth cylindrical surfaces spaced apart sothat both sides of a given area oflin areneyerincontact with two Vsuchsurfaces''atfthe sametimaat least the intermediate'two'of which arejlatgindependently controlled temperatur' jdf atleast 200 F., the areas of contact between 'theflrirbeing treated and the respective'fsrrioothlcylindrical heated surfaces being chosen f" to proyide'.. 'an vaggregate contact ltimeon'saidheatedsuriaces ofwfr'm 1.5 to 20, and

preferably from@ to 6.5 fs'econds. The range offuseful' temperatures'jatfeach of the four successive` pointsgioif con-tact between the iilm f cylindrical'jsurfaces are, re-

yat"this"pointit is more imline vinylidene chloride copolymer film is fed at a steady rate from a source (not shown), which may-.be a supply reel orfmayfV be the.; production process of Stephensons U. S. Patent No. 2,452,080, and is passed successively over each of four smooth cylindrical rollers and thence to a Windupfreel-(notshow-nl., Tension is maintained on when separated therefrom. Ille .nal.cy1indrical. 1 0- contact surface may be unheated.;as.,iiisagain, more important that the lmfpassingfg-this surface,.- be held smoothly and under tension than it is to attempt any further slight shrinkage. It is desirable that the nal contact surface should'i'note Vther-161m; bwa-ny-k'nown means, such as auxiliary snubber rollswhichiare not illustrated. The four cylindrical rollsy may be of identical size and may be, driven., at,'identical peripheral speeds by a i singlexdrive... 'Illieyinay have different diameters be heated higher than about 240 F., andeinno event must it be as hot as the two intermediate contact.surfac,es, or. the; film leaving the fourstgeLteating; zonefwi'llibe" toowrinkl'ed -and un:- evenf. 'Ifefoursaid cylindrical .surfaces may 'be causeiilth lctate. 'ati the" same' peripheral speed'si but. sin'ce.the.ling`lieingg treated tends'to shrink; snxrxewli'atl during, the? treatment'it may; emerge; from tthe treatinglon atf a; somewhat-lower' ratei than-:.tlie rate. ofjfeedtojthe treating; zone. Ace` cordingly.' it`hasbeen lfounddesirableto .driveth'e respectivecontact surfaces'. ati' diminishing pee riplierall' speeds., Th"us", the'flr'sttwo cylindrical' surfaces; mayhaverth'ez same: peripheralspeeds, andi the' last', twor suc'h surfaces' and? the product; takefoi roll.- may have lesser peripheral: speeds: down toahoutlSpcr:centofithespeed lof the iirst two. contactLsurfaces aswilllleillil'stratedn examplsitb. follow. y

While. the; presenti invention desA not. require.v the use of any particular apparatus, and, the( various types. 0i. equipmentwhichmay b'eempleyedvformfnosparto'the.presentlinventiom ,the

Y processmayfbe.understoodmorereadilywhen.def

scribed with rerencetq the accompany-ingdraw.-A ingggther single-.diagrammatic gurefof which. il.,- lustratesffl -i1r ,verti cal 1. cross-,section a! series. of; smoothcylindnioal rollersior: use carrying out, the presentfmethed: YThe legends. on :the drawing" indicatav preferred.V operatingconditions.

Reierringfto;the-drawings.amorientedcrystale andthusghave-diftrent peripheral speeds within the limits stated on the drawing. 'I'he various rolls-'mayfldedrivenlindividually so that, regardlessofrtheir relative sizes, the peripheral speeds may each be adjusted t0.,within the ratios indicated on. the drawing. The rolls may be stacked somewhat.closer tcgetherf than indicated in-` the drawing, to increase-theearea ofAv contact-loetvveenv each' roll'and'the*fliin` being-treated', or theymay be. spacedgfarther apart* to decreasethe; contact areasY and,4 hence; 'theperio'ds'l off exposure ofthe lm'tothe temperaturesofi'the respective rolls.:l

Therollsareheatedindividually byA any of; several? known4` means; y 'Ilieintroduction of-I steam under:.carefully-controlledpressuresVv through the axisjofj.` eachhollowfrolleris-a convenientmethod of "'eiiecting therequiredj temperature 'control-.f

In a `speciflc example; affilm- 1i mil'A thick, composed of a' plasticizedv copolymerconta'ining about 92p er cent*vi'nylidene` chloride and 8 per cent vinyl chloride, byl'weight, in--thejcopolymer molecule,v Was` passed: over the illustrated rollers at the, speeds;and exposedto the' temperatures given i'n ,the followingt'able. The film` was kept; under theyt were? 'remeasuredf andlresidual' shrinkage was. computedl per.. centi 0i" the corresponding.

dimensiomaiterf heattreatinentii buthefore the shrinkagegtest. A`n` increase. dimension .in the, 10 minute, 212"7`E4tes'1,isrecorded. as.- a negative .1 residual shrinkage... In room tempera'tur'e,v

the..talile, meansv man@ It is observed from theforegoing data that the exposure of the crystalline'iilm to `a single heated contact surface is insufficient" 'to' *reduce the residual shrinkage value to lthejdesired` range of per cent or less. It is noted, also, that the preferred operating conditions are those previously recited and V.that optimumA resiili's",v are lobtained when the speeds of the third andfourthrotating contact 'surfaces are somewhat vless th'an -those of the first and second such surfaces. In no instances, reported in the table or discovered in numerous other tests, has the desired result been obtained when the aggregate priod of contact between the crystalline film and the various heated rolls has been in excess of seconds. While untreated iilm has the property of shrinking from 20-30 per cent in both major directions when heated to 212 F. for 10 seconds, the film which has been treated in accordance with the present method and under the recited preferred conditions is seen to have shrinkage values near and usually less than 5 per cent in both major directions.

Reference has been made to the maintenance of tension on the film during treatment. The tension maintained should be sufficient to keep the film free from wrinkles and to advance it continuously through the treating zone but should be insufficient to cause actual stretching of the film. For this reason the codnitions used in runs Nos. 16-30 inclusive, in the foregoing table represent the preferred type of operation.

For convenience and for ready comparison between the results in the several tests, the foregoing table of data is based upon the use of a particular crystalline vinylidene chloride copolymer film. Experience has shown that the invention is not limited to this specific film'but is equally useful with any oriented crystalline film of a vinylidene chloride copolymer. Numerous such copolymers are known and are commercially available. These may be made into films by the method described in the above identified Stephenson Patent No. 2,452,080. The compositions from which the crystalline and oriented films are made usually contain small amounts of a plasticizer to facilitate extruding the composition in film form and small amounts of other materials normally added to y stabilize the polymer composition against the destructive effects of heat during the extrusion operation. The aggregate of such addition agents is usually no more than 10-12 per cent of the composition and, since these are standard constituents of the films here concerned, it is not deemed necessary to specify their presence in the films recited in the appended claims.

There is filed concurrently herewith an application of Walter A. Klein, Robert R. Trull, and Edward V. Dettmer entitled, .Apparatus for Preshrinking Crystalline Vinylidene Chloride Copolymer Film, now U. S. Patent 2,540,986. The apparatus described in the said patent is one of the ones which may be adapted for use in carrying out the process of the present invention.

We claim:

1. The method which comprises supplying continuously and at a steady linear rate an oriented crystalline vinylidene chloride copolymer film maintained under tension sufficient to advance the lm in a wrinkle-free condition but insufficient to stretch the film, and applying the moving film successively to, by advancing it over, each of four non-contiguous rotating smooth cylindrical surfaces which are spaced apart so that both sides of a given area of film are never in con- 6l. tact with two s1ichsu'rfa' y s least the second andj-thirdsuch' surfaces being at independently controlled Vtemper atures of at least 200` F. but ynot to eiceed265- F., thel first said"v surfacehaving Y,a temperature between room* temperature and 220 F. which temperature is less than those of the second and third'saidjsurfaces, and the fourthsaid surface being at a y temperature from*rooirif'temperature to 240 F.

which is also less" than those of the second and third said surfaces, the areas of contact between the moving film and the various heated cylindrical surfaces and the rate of advance of the film over said surfaces being adjusted to provide an aggregate contact time between the fihn and the heated surfaces of from 1.5 to 20 seconds, and withdrawing continuously the so-treated film from the treating zone.

2. The method which comprises supplying continuously and at a steady linear rate an oriented crystalline vinylidene chloride copolymer film maintained under tension sufcient to advance the film in a wrinkle-free condition but insufficient to stretch the film, and applying the moving film successively to, by advancing it over, each of four non-contiguous rotating smooth cylindrical surfaces which are spaced apart so that both sides of a given area of film are never in contact with two such surfaces at the same time, at least the second and third such surfaces being at independently controlled temperatures of at least 200 F. but not to exceed y265 F., the first said surface having a temperature between room temperature and 220 F. which temperature is less than those of the second and third said surfaces, and the fourth said surface being at a temperature from room temperature to 240 F. which is also less than those of the second and third said surfaces, the areas of contact between the moving film and the various heated cylindrical surfaces and the rate of advance of the film over said surfaces being adjusted to provide an aggregate contact time between the film and the heated surfaces of from 2 to 6 seconds, and withdrawing continuously the so-treated film from the treating zone. Y Y,

3. The method which comprises supplying continuously and at 'a steady linear rate an oriented crystalline vinylidene chloride copolymer film maintained under tension sufficient to advance the film in a wrinkle-free condition but insufficient to stretch the film, and applying the moving film successively to, by advancing it over, each of four non-contiguousrotating smooth cylindrical surfaces which are spaced apart so that both sides of a given area of film are never in Contact with two such surfaces at the same time, the first of which surfaces is at a temperature from room temperature to F. inclusive, the second of which is at a temperature from 215-260 F., the third of which is at a temperature from 250-265 F., and the fourth of which is at a temperature from 210-240 F., the areas of contact between the moving film and the various heated cylindrical surfaces and the rate of advance of the film over said surfaces being adjusted to provide an aggregate contact time between the lmand the heated surfaces of from 2 to 6 seconds, and withdrawing continuously the so-treated film from the treating zone.

4. The method as claimed in claim 3 in which the peripheral speeds of the third and fourth ro s "aethesame time, lat- 

1. THE METHOD WHICH COMPRISES SUPPLYING CONTINUOUSLY AND AT A STEADY LINEAR RATE AN ORIENTED CRYSTALLINE VINYLIDENE CHLORIDE COPOLYMER FILM MAINTAINED UNDER TENSION SUFFICIENT TO ADVANCE THE FILM IN A WRINKLE-FREE CONDITION BUT INSUFFICIENT TO STRETCH THE FILM, AND APPLYING THE MOVING FILM SUCCESSIVELY TO, BY ADVANCING IT OVER, EACH OF FOUR NON-CONTIGUOUS ROTATING SMOOTH CYLINDRICAL SURFACES WHICH ARE SPACED APART SO THAT BOTH SIDES OF A GIVEN AREA OF FILM ARE NEVER IN CONTACT WITH TWO SUCH SURFACES AT THE SAME TIME, AT LEAST THE SECOND AND THIRD SUCH SURFACES BEING AT INDEPENDENTLY CONTROLLED TEMPERATURES OF AT LEAST 200* F. BUT NOT TO EXCEED 265* F., THE FIRST SAID SURFACE HAVING A TEMPERATURE BETWEEN ROOM TEMPERATURE AND 220* F. WHICH TEMPERATURE IS LESS THAN THOSE OF THE SECOND AND THIRD SAID SURFACES, AND THE FOURTH SAID SURFACE BEING AT A TEMPERATURE FROM ROOM TEMPERATURE TO 240* F. WHICH IS ALSO LESS THAN THOSE OF THE SECOND AND THIRD SAID SURFACES, THE AREAS OF CONTACT BETWEEN THE MOVING FILM AND THE VARIOUS HEATED CYLINDRICAL SURFACES AND THE RATE OF ADVANCE OF THE FILM OVER SAID SURFACES BEING ADJUSTED TO PROVIDE AN AGGREGATED CONTACT TIME BETWEEN THE FILM AND THE HEATED SURFACES OF FROM 1.5 TO 20 SECONDS, AND WITHDRAWING CONTINUOUSLY THE SO-TREATED FILM FROM THE TREATING ZONE. 